Recent demands on wireless and mobile communications motivate exploring new technologies to improve network performance in terms of overall throughput, spectrum utilization, energy consumption and so on. Meanwhile, the appearance of new commercial services such as location-based services and content sharing services encourage us to explore new paradigms to meet user demands. Device-to-Device (D2D) communications were proposed as one of the promising technologies for communications in the vicinity, which is supposed to play as a key role in the next generation mobile communication networks and wireless systems (i.e., 5G).
D2D communications refer to a type of technology that enables devices in the vicinity to communicate directly with each other under the control of existing network infrastructures such as Access Point (AP), Base Station (BS) and Core Networks (CN). It has shown great potential in improving communication capability, reducing communication delay and power dissipation, as well as fostering multifarious new applications and services. As a promising technology, D2D communications have drawn considerable attention in academia, industry and standard organizations in recent years. In academia, D2D communications were regarded as an underlay of LTE-Advanced network to improve communication performance. Many researchers have paid their attention on application scenarios, communication mode selection, resource allocation, power control and interference control. In industry, D2D communication-based application developments are active. For example, Qualcomm is developing a D2D communication sub-system in cellular networks, known as FlashLinQ, to make the communications among proximity devices possible. It is expected to complement traditional cellular networking based services and serve as a scalable platform for new types of applications, such as advertising, content sharing, and secure mobile payments. At the same time, the standardization work on D2D communications is on-going in standardization communities. The Third Generation Partnership Project (3GPP) is one of them. In 3GPP, D2D communication is defined as Proximity-Based Services (ProSe). Many technical issues are under discussion in the aspects of the feasibility study of the ProSe in LTE-A, system architecture and network entity functions, as well as extensive use case exploration. The above issues and more will be discussed and standardized over the next few years. Moreover, the studies on radio, service and other aspects are underway and will appear in future standardization.
In spite of the significant benefits, new application scenarios and system architecture expose D2D communications into unique security threats. Whether we can provide a secure and guaranteed D2D communication system is essential for the success of D2D services. One basic but significant open security issue is to establish a secure communication channel between a number of D2D devices.